WARNING: This page is about a different car, the 2006 Volkswagen Jetta and 2005 Volkswagen Jetta. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: Identifying Ceramic Sheathed Element Glow Plugs & Metal Glow Plugs
Courtesy of VOLKSWAGEN GROUP OF AMERICA, INC.
A - Ceramic glow plug
Phased stepping on tip (see magnified area) color coding -arrow- : white